cnc machines

COMPUTER NUMERICAL CONTROL MACHINES

CONTENTS

1.             CONSTRUCTIONAL FEUTURES OF CNC

2.             WORKING PRINCIPLE OF CNC

3.             KEYS OF CNC

4.             TYPES OF CNC

5.             SAFETY CHEKING ON CNC

6.             PROGRAMMING OF CNC

7.             FUNDEMENTALS OF CNC PROGRAM

8.             PROGRAM STRUCTURE

9.             A PROGRAM

10.                       CO-ORDINATE SYSTEM

11.                       ADVANTAGES OF CNC MACHINES

Computer numerical control machines

INTRODUCTION

Computer numerical control machine is a numerical control system that utilizes a dedicated, stored program computer to perform some or all of the basic numerical control functions. because of the trained towards down sizing in computers, most of the CNC systems solved today used micro computer based to control the unit over the years mini computers have also been used in CNC controllers

                       The external appearance of a CNC machine is very similar to that of a conventional NC machines. Part programs are initially entered in a similar manner punched tap readers or still the common device to input the part program into the system .In CNC, the program is entered once and then stored in the computer memory .Thus the tap reader is used only for the original loading of the part program than as compared to the regular NC. CNC offers the additional flexibility and computational capability.  A new systems option can be co-operated into the CNC controller simply by reprogramming the unit because of the reprogramming capacity. Both in terms of part program and system control option CNC is often referred to y the terms soft wired numerical control machine 

CNC LATHE MACHINES

           

                             

Constructional features’ of CNC machine tools

CNC machine tools differ from conventional machine tool in a number of respects. The special constructional features of CNC machine include rigid structural design, infinitely variables spindle and feed drive , slide ways fitted with plastic inserts or linear bearings for axis drive, automatic tool changers, chip conveyors special rigid tool turret etc.

In order to take care of above and many other factors, there is a need for special consideration to be given to the design of CNC machines tools in the following area.

Machine structure

To meet the requirements of high precision repeatability and high efficiency the numerically controlled machine tools should have a structure that is correctly designed to withstand normal weight distribution. The higher cutting speeds and feeds in CNC machine results in rapid acceleration and deceleration of the slides and the machines are subjected to fluctuating and variables forces during the machining operation. Also the machine structure design should be such that removal of sward is easy and the chips do not on the slide ways.

Slide ways

Positioning and repeatability of machine tool slide are the major functional requirements of CNC machines. The inaccuracies that caused are mainly due to the stock slip motion when plain slide ways are used. Stick slip is a phenomenon which occurs when small movements between two lubricated elements are required. Lubricating medium tries to cause the mating elements to stick to each other to resist motion and result in a jerky motion as the mating elements try to stick and then to slip during  their relative movement.

To elimination of the requirements of elimination of stick slip there are different slide way system such there are different slide way system such as rolling function slide ways are used they are low wear, no stick and low price .To meet these requirements in CNC machine tool slide ways the techniques used include

v Linear motion bearings with balls rollers or needles.

v Plastic coated slide ways.

v Hydrostatic slide ways.

Linear motion bearings

The sliding friction due to direct metal to metal contact between the slides and slide ways is replaced with rolling friction by the use of antifriction ball or bearings. Rolling element when applied to reciprocating motion has followed advantages.

ü Reduce the drive power to be used and also makes the drive equipment more compact.

ü Stick slip problem is completely eliminated.

ü Require only small amount of lubricant.

ü Absorb the machine vibration.

Plastic coated slide ways

The linear motion bearings have a lesser load bearing properties and damping

Hydrostatic slide ways

In the hydrostatic slide air or oil is pumped into a small pockets or cavities machined into the carriage on slides which are in contact with the slide ways in this the surface of the slide member is supported from the slide way by continuous supply of very thin film of fluid. The hydrostatic slide way provides almost a frictionless condition for the movement of the slide. For efficient operation it is very important that the fluid and slide ways are kept clean. Also the hydrostatic slide ways need a very large surface area to provide adequate support.

Measuring systems

In CNC machines the electronic measuring systems are used on each controlled axis and to monitor movement and compare the position of the slide and the spindle.

To Measuring system used in the CNC machines are of two types

Ø To measure the position of the slide (linear measurement )

Ø measure the spindle speed (rotary measurement)

A linear scale is used as a linear measuring device which is used very often. In this method linear displacement is measured directly at the slide. The measuring device is fixed on to the moving machine element, which detect the actual distance traveled by the machine slide.

One of the rotary measuring devices is an incremental rotary encoder.

Incremental measurement means measurement by counting. The out put signal of incremental rotary encoders is fed to an electronic counter which provides the corresponding values.

Work holding device and tooling

The CNC machines are capable of performing a number of operations using different tools, on different faces of the component in a single setting. This requires that the component should be accessible from different sides without changing of clamps or repositioning of components. The multidirectional cutting forces to reduce the clamping and unclamping time, hydraulic and pneumatic actuation are widely used in work holding devices.

Hydraulic chuck is commonly used in chucking operations in CNC lathes.

A work holding devices for a CNC application should have the following function.

Ø It should restrict the linear and rotary motion of the component.

Ø The component should nit distort or deflect due to cutting forces in the process.

Ø It should facilitate quick loading and unloading of the component.

Ø It should not interfere with the cutting tools.

Ø It should provide easy removal of chips

Ø It should be safe

Automatic tool changers

The CNC machines are designed to perform a number of operations in a single setting of the work-piece. To reduce the down time in change over from one operation to the next, the CNC machines are equipped with automatic tool changing facility.

Turning centers are available with the turret containing 8-12 tools. As the tool change command is received by the control system, the tool turrets moves to a fixed tool change position by rotating the tool turret.

On the machining centers (milling centers) a separate ATC (Automatic Tool Changers) is provided to change over the tool by the control system itself.

The ATC consist of a tool magazine for storing the tools and tool change unit for transferring the tool from tool magazine to the spindle. As the tool change command is received by the control system, the present tool released by the tool change unit from the spindle and the required tool is fitted to the spindle.

WORKING PRINCIPLE OF CNC MACHINES

Keys of cnc machine

A cnc machine contain following keys for the smooth function of the machine

A key board is provided for the following purposes.

1.     Editing of part program

2.     Selection of different pages

3.     Selection of operating modes

4.     Execution of part program

5.     Selection of speed and feed

 

Control  keys

 

Editing and input keys

 

1.Delete message   2.Delete word      3.Modify word    4.Input character

Operating keys in machine control panel are discribed below

 

                  Reset key

			Start

 

                 Program start

			Stop

 

                 Program stop

+Y

+X

+Z

-Z

-Y

-X

 

Axis direction keys/ jogging keys

 

              Operating mode selection key

Manual mode, Automatic mode, Input mode, Reference mode are here

Editing keys

Types of CNC machines

There are mainly two types of CNC machine

Ø Machining centers

Ø Turning centers

(1) Machining centers

There are very important types of CNC machine and are multi-functioning machine equipped with automatic tool changes and are capable to carrying out, milling, and drilling, reaming, taping, boring, counter boring

  In this type of machines there are three types of axis X axis, Y axis, Z axis. Here the tools are rotated clockwise or anticlockwise the works still fixed. 

Turning centers/ CNC lathes

CNC lathes have only two axes X and Z, X axis for longitudinal movement and Z axis for crosswise movement. Tools are arranged in turret with a number of stations some lathes have rotating tools in the turret to facilitate off-axis machining such as drilling, milling etc

These machines have additional controller for spindle rotation such machines are known as turning centers

Safety check on the CNC machine

Ø Check the main pressure of the hydraulic and pneumatic system

Ø Check the clamping pressure of the hydraulic and pneumatic system.

Ø Check the function of key lubricates the clamping device. If working in manually move in reference point in return mode.

Ø Check the sliding movement

Ø Check the tool bits

Ø Check the tool position with respect to the program

Ø Check the tool indexing safety while the machine is automatic tool changer

Ø Check the program control levels try to cut job in a single block

Ø Do not use compressed air for cleaning.

Ø Open the door at the end of the job in jog mode.

Ø Check the coolant supply by pressing appropriate switch manually.

Ø Check the speed over the ride position.

PROGRAMMING OF CNC MACHINES

CNC programming is a term that refers to the methods for generating the instruction that drive the CNC machine tool. Part program is a set of instructions which instructs the machine tool about the processing step to be performed for the manufacture of a component.

There are two distinct techniques adopted for creating CNC programs.

                                                       i.            Manual programming

                                                    ii.            Computer assisted programming

1.     Manual programming

A program is a series of instructions designed to produce a work piece on a CNC machine. These instructions are supplied to the machine controller using a coded format. The controller interprets these instructions and delivers control signals to the various drive units of the machine as required. A part program is designated to control.

1.     Positioning

 A dimensional instruction that move and position the tools or work pieces to produces the required work

2.     Motion

The relative velocity of the tool with respect to the work piece is to be controlled, in both spindle drive and positioning drive

3. Switching function: - The auxiliary function of the machine such as coolant, tool changes and program stop.

The following should take into account when writing a part program.

Ø The machine features and capacity.

Ø The size if the component.

Ø The material to be machined.

Ø Machine co-ordinates.

Ø Component datum.

Ø The sequence of operation.

Ø The tooling to be used.

Ø Component holding and location.

It is important that all programs are checked and proved before machining takes place. Programs are initially written on the paper using codes consisting of alphabets and numbers. Before machining the part the completed program has to be fed into the computer memory of the machine and this can be done in different ways

1.     Manual data input through the keyboard of the machine

2.     Using the floppy and disc

3.     DNC mode: Directly from the computer used for programming or storage of program to the memory of the CNC machine.

2.     Computer Aided part programming

CNC machines of complex 3 dimensional components require thousands of instructions and with out computer assistance it would be impractical to write part programs for the production of such components. NC part programming languages and the processors and post processors that translate the language instruction into NC programs.

 There are several NC languages in use. Some of them are

            v   APT

            v   ADAPT

            v   EXAPT

            v   UNIPAT

            v   SPLIT

            v   PROMPT

FUNDAMENTALS OF PART PROGRAMMING

A program is a set of encoded information giving coordinate values and other details to indicate how a tool should be moved in relation to a work-piece in order to achieve a desired machining form.

The program is prepared by listing the coordinate values (XYZ) of the entire tool paths as suited to machine the complete the component. The coordinate values are prefixed with preparatory codes to indicate the type of movement required (point to point, straight or circular) from one coordinate to another. Also, the coordinates are suffixed with miscellaneous codes for initiating machine tool functions like start, stop, and spindle movements, coolant ON/OFF etc.

In addition to these coded functions, spindle speeds, feeds and the required tool numbers to perform machining in a desired sequence are also given. All these elements represent a line of information and form one meaningful command for the machine to execute and is called a block of information. The number of such blocks written sequentially forms a part program for the particular component.

A part programmer has to do the following while write a program

Ø    Study the drawing of component thoroughly.

Ø    Identify the type of material to be used

Ø    Determine the specification and functions of machine to be used.

Ø    Decide the dimensioning units metric or inch.

Ø    Decide the coordinate system absolute or incremental.

Ø    Determine the cutting parameters for the job/tool combination.

Ø    Decide the position of work zero.

Ø    Checks the tooling required, and take tool offset for each.

Ø    Establish the sequence of machining operation.

PROGRAM STRUCTURE

In manual part programming the programmer has to determine the machining parameters and the sequence of operations to be performed. Based upon this sequence, the tool path is calculated and program is written. The basic unit of a program input to the control is called a block. The term block structure of format refers to the rules for writing a program block.

A block contains any or all of the following.

Ø Sequence or block number ( N)

Ø Preparatory function(G)

Ø Dimensional information (XYZ etc)

Ø Decimal point (.)

Ø Feed rate(F)

Ø Spindle speed (S)

Ø Tool number (T)

Ø Tool offset function(D)

Ø Miscellaneous function(M)

Ø End of block(EOB/*)

Program block and program word

A block contains all data required to implement an operation procedure. The block comprises several words and the block end character “* “

Each line of the program is called a blocks called program words. The alphabets in these words are called address letters.

A word is an element of a block. It comprises an address character and a string of digits. The address character is normally letter. The string of digits may be specified with a sign and with decimal points. The sign is written between the address letters and the string of digits. A positive sign may be omitted.

Sequence number ( N-word )

The first word in every block is the sequence number. The sequence number is used to identify the block. The sequence number is preceded by word N and is written as N01, N02 etc. The program is executed from lowest block number to highest unless instructed otherwise. It is customary to start with block number 10 and precede in steps of 5 or 10, so that accidentally omitted block may be  inserted easily.

Preparatory function (G-words )

 They are the commands, which prepare the machine for different modes of movement like positioning, contouring, thread cutting etc and for presetting the machine.

Function	Meaning
G00	Point to point positioning
G01	Leaner interpolation
G02	Circular interpolation
G03	Circular interpolation (Anti clockwise)
G70	Dimensioning in inch
G71	Dimensioning in mm
G90	Absolute dimensioning
G91	Incremental dimensioning
G96	Constant surface speed
G97	Spindle speed

Miscellaneous function

The miscellaneous function word is used to specify certain miscellaneous or auxiliary functions which do not relate to the dimensional movements of the machine. The miscellaneous functions may be spindle start, spindle stop, coolant ON/OFF, etc.

Function	Meaning
M00	Program stop
M01	Optional stop
MO5	Spindle stop
MO6	Tool turret index
M09	Coolant off
M10	Clamp work piece
M11	Release work piece
M30	End of program

A CNC Program for making this component

 

            

              20 cm         10cm       5cm

% 100

N10 G90 G71 G95 LF

N20 G00 G53 Do LF

N30 G54 M03 S600 LF

N40 G00 X80 Z-35 LF

N50 G01 X60 F1.2 LF

N60 G01 Z-15 LF

N70 G01 X4O LF

N80 G01 Z-5 LF

N90 G01X20 LF

N100 G01 Z0.00 LF

N110 G01 X0.00 LF

N120 M09 M05 G53 M30 LF

METHODS OF LISTING THE COORDINATES OF POINTS

The coordinate systems are used to define and control the position of the tool in relation to the work piece. Each system has its own applications and the coordinate systems may be used indecently or may be mixed, within a CNC part program according to the machining requirements of the component. The co-ordinate systems used in CNC machines are as follows.

a)    Absolute co-ordinates

b)    Incremental co-ordinates

Absolute co-ordinates

In the absolute co-ordinate system the co-ordinates of a point are always referred with reference to the same datum. The datum position in X axis and Y-axis and Z axis are defined by the program before starting the operation of the machine.

A major advantage of this system is that it is very easy to check and correct a program written is made in the value of any dimension in a particular block.

 

               20           35           80

 

Incremental co-ordinate

In the incremental co-ordinate system the co-ordinates of the any point are calculated with reference to the previous point i.e., the point at which the cutting tool is positioned is taken as a datum point for calculating the co-ordinates of the next point to which movement is to be made.

It is difficult to check a part program written in incremental dimension in a particular block, it will affect subsequent blocks.

 

                                                                                                                              .                        20           15           25     

Advantages of Computer Numerical

Control Machines

Ø Accuracy and repeatability is very high as compared to the NC machines.

Ø Reduced scrap and rework.

Ø Reduced inspection time

Ø Reduced material handling

Ø Easy of inter changeability of machine parts

Ø Easy editing time than NC machines

Ø Complicated shapes and curves are easily manufactured with quality assurance and better production.

Ø Reduced skilled operators can work in CNC machines

Ø It can work 24 hours a day because the resting times are very low

Ø Time taken to complete the works is minimum as compared to manual machines

Ø Program editing at the machine site